The invention relates to a suction feeder for glass processing, with a feed line of noble metal or a noble metal alloy which is heatable in operation and is embedded, in thermally insulated fashion, in a housing that has a connection element for connection with a delivery device for molten glass and an outlet element for operative engagement by a suction element, in particular a suction ball, that takes a gob of glass from the feed line.
In the production and processing of glass, molten gobs of glass are typically needed as the starting product. In most glass processing facilities, the glass is taken by glassmakers from an open surface, from so-called front fixtures. However, it is also known for the gobs of glass to be taken from droplet feeders or suction feeders. In these suction feeders, to which the invention draws its attention, the gob of glass is generated from a melt by suction. These suction feeders are used in both the channel region and the feeder region.
In FIGS. 2 and 3, two typical known suction feeders are shown. The suction feeder of FIG. 2 comprises a tube 1, made from a noble metal (alloy), which is surrounded by a plurality of layers 2, 3 of refractory material, that is, a backing composition 2 and an insulation 3. Together, they are built into a steel chest 4, which lends the overall structure the requisite stability. The noble metal tube 2 itself is heated directly with the aid of alternating current, in that the requisite heating current is fed in via two or more flanges 5, 6 (depending on the length of the assembly). As a rule, this noble metal tube 2 is curved upward by 90°. The suction feeder is designed such that the level of glass in the entire system is just at the upper edge of the upper flange 6. As a rule, so-called suction rings are also placed on this flange 6, the diameter of these rings being selected as a function of the range of articles involved. In this technology, the possible vaporization of the glass at hot open surfaces is reduced to a minimum.
A so-called suction ball 7, whose diameter is also dependent on the article involved, is placed on the upper flange 6, or on the associated suction ring, and with the aid of a predefined negative pressure, this ball aspirates the glass from the noble metal tube 2, which is what has led to its being called a “suction feeder”. Once the suction ball 7, after a predetermined course of pressure and time, is filled with glass, it is lifted from the suction feeder, and the gob of glass contained in it is carried elsewhere for further processing. As a rule, the further processing is done by glassmakers, and the gob of glass is either blown up or compressed.
The suction feeder can be positioned at any arbitrary point on an existing glass distribution and channel system. The attachment is done either at a so-called brick part 8 with a corresponding through bore, or directly to a platinum channel.
The curved noble metal tube 1 has a typical diameter of about 120–300 mm, for a length of approximately 500–1000 mm; usually, it tapers conically from the entrance site of the glass to the outlet. The magnitude of the diameter is dependent on the desired range of articles, and discontinuous withdrawal must be taken into account along with the requisite heating capacity (process temperatures and heat balance in the apparatus).
For small gobs (articles) of glass that can still readily be picked up by glassmakers (≦=15 kg), the suction feeder of FIG. 2 is as a rule used. For larger gobs (up to 150 kg, again with processing by a glassmaker using a special machine), a suction feeder of FIG. 3 is used, that is, a combination of a container 9 (so-called bowl) made of refractory bricks, and a noble metal cone 10 seated on the container. The noble metal cone 10 is again heated directly; the brick container 9 is heated via a electrode heater that heats the glass directly.
Compared to the still widespread technique today of withdrawing glass from front fixtures, in suction feeders, by the use of noble metals (noble metal alloys), the temperatures required for the process can be established better and more stably. However, if there is a refractory lining of the feeder, then because the heater heats the volume of glass asymmetrically, it can happen that glass of different temperatures is united in one article, causing so-called “cold streaking”, which can be avoided by means of a suitable, adapted temperature control. Bits of brick, streaking, or bubbles resulting from corrosion of the refractory material are also found in the product. These bubbles originate in the corrosion of the refractory material itself.
The avoidance of bubbles, which pass through porous refractory materials from outside into the glass melt, is described in a German Patent DE 42 02 278 C2. The bubble problem caused by porous refractory materials can be avoided if only very dense material (with stability to corrosion at high temperatures) is used in contact with the glass.
In practice, still other inadequacies, which can have an extremely adverse effect on the glass quality, are linked with the above-described known suction feeders shown in FIGS. 2 and 3.
The first problem is the stability of the noble metal components used, that is, the tube 1 and cone 10:
In use in the glass melt, a maximum temperature must not be exceeded. The closer one gets to the melting temperature of the applicable material, the lower is its mechanical stability. At the typical temperatures in use in the glass melt (at least for special glasses, which require a comparatively high temperature), platinum becomes soft and then has a consistency similar to paper. The mechanical stability then permits only a very limited duration of use. To improve the mechanical stability, the noble metal tubes are provided with so-called beads. Additionally attached reinforcing rings, which are anchored in the refractory material behind them, provide additional stability, but this has an adverse effect on the heat balance.
By means of the negative pressure applied to the glass side during the suction process in the interior of the noble metal component 1 and 10—the buildup and reduction of the negative pressure takes place within only seconds—the noble metal envelope is exposed at periodic intervals to severe pressure fluctuations, since atmospheric pressure always prevails on the outside of the noble metal envelope and particularly in the vicinity of the suction ball 7 can cause the component to collapse. This changes the temperature conditions, and as a restricts the throughput so severely restricted that production is no longer possible. This problem becomes even more serious at weld seams and with long use, since the material is additionally made brittle by the high temperatures and the glass attack, and the initial stability is severely reduced. Thus until now, only suction feeders with relatively small diameters have been used.
The second problem is aspirated air bubbles:
The suction feeder unit is located directly at the point of transfer to postprocessing, so that at this position there is no longer any opportunity of removing existing bubbles from the glass melt.
Above all, in the embodiment of FIG. 3, there are several critical positions where air bubbles can be aspirated from outside because of the negative pressure prevailing in the glass composition. On the one hand, these are the connections between the noble metal cone 10 and the fireproof bowl 9 and the feed-throughs for the thermocouples in the bowl, where cracks can develop, along with the bores for the heating electrodes. Although glass is capable of penetrating into seams and cracks, thereby sealing them, still the course of the temperature and negative pressure differs from one article to another, making for unstable sealing or vitrification and allowing the aforementioned aspiration of air bubbles.
Since as a rule the noble metal components 1, 10 are embedded in a backing composition 2 of a ceramic material, known by the tradename “Quarzal”, which because of the curing process is not completely leakproof, air can also be aspirated through cracks occurring in these noble metal components as a consequence of embrittlement from long usage or from chemical or mechanical attack.